According to the Norsok-requirements (D-SR-007), which are set in connection with well testing, it is required to have a check valve installed in between the kill valve on the surface test tree and the cement unit of the rig.
The reason for the requirement of a check valve is as follows; if the well has to be killed during the test-operation, the kill valve on the surface test tree must be opened so that well-killing mud can be pumped from the cement unit through the kill valve and down into the well. The moment the kill valve is opened, said check valve will prevent hydrocarbons from flowing down towards the cement unit. It is extremely dangerous to get hydrocarbons, in the form of gas, into a cement unit.
Said check valve, commonly a flapper valve, has until now had a manual mechanical override function. A threaded steel rod is screwed into the valve housing and pushes the flapper off the valve seat and leaves a passage through which the fluid can be pumped or bled off.
The override function is necessary to allow for pressure testing, and accompanying bleeding of the test tube string, according to programs and procedures for testing of wells.
When the test tubing string has been verified as being good, the following operation is to perforate the well and to subsequently flow hydrocarbons to the surface.
However, before the test tubing string is set under pressure, the manual mechanical override function must be disengaged so that the check valve is operating as intended, in other words preventing back flow from the test tubing string to the cement unit. Disengagement of the override function is obtained by screwing the above mentioned steel rod out of position and off the flapper element, thereby allowing the flapper to seal against the valve seat.
The check valve is barrier number two, after the kill valve, on the surface test tree, i.e. the check valve shall function as a barrier against the cement unit in the case of the kill valve not being able to keep itself sealed, see the explanation above. Thus, from this point in the operation program, the mechanical override function is not to be in operation.
In order to disengage this mechanical override function, personnel must move up in the ride belt and manually disengage the function by using a threaded rod and a wrench.
Thus, this operation requires personnel to walk in the ride belt and to work within the so-called defined red zone. In principle one is not allowed to plan for operations in the red zone. However, at the present it is not possible to avoid this operation. Accordingly, such operations require an internal deviation from standard every time it is to be performed. It is always a risk of falling objects when work is performed in the ride belt, and if the sea in addition is rough it is a dangerous operation. It requires approximately 30 minutes of operational time each time the override function of this check valve is to be disengaged or engaged.
As described, the present solution for a check valve in the surface test tree leads to an increased risk for the operational personnel who are required to move around in the ride belt in order to disengage or engage the override function of the check valve. In addition, such a manual function leads to a long operational time; approximately 30 minutes each time the override function is disengaged or engaged. In this time period other operations must be set on hold while personnel are present in the red zone.
A further problem with the present check valves is leakage through the check valve when there is zero or low pressure on the well side.
Thus, in the presently known technique there exists a pressing need for a solution which can both reduce the risk towards operation personnel, and shorten the operational time, in connection with the disengagement and engagement of the override function of the check valve installed in between the kill valve and the cement unit of a surface test tree. In addition, it is desirable to minimize or remove leakage in connection with zero or low pressure on the well side of the valve.
At the present there are no check valves having a hydraulic override and which are suitable for the use described above. In the literature, a number of check valves for use in for example drill strings are described. However, these valves do not fulfill the necessary requirements concerning size (short length) and adequate sealing of the valve at low/zero pressure on the well side. Common features of check valves suitable for use in drill strings are that they do not have any restrictions regarding their length, and that they are designed primarily to be in an open position.
EP 0985798 describes a check valve suitable for use in a drill string. This valve is intended to be open during a normal operation, and is being closed in the case of for instance a blowout. In order to obtain a proper sealing, the valve is dependent on an adequate amount of excess pressure on the well side to push the flapper against the valve seat. The design of the power transfer from the hydraulic/spring-system is not suited to provide an even and sufficiently high pressure on the flapper such that the valve is tightly closed without the help of excess pressure on the well side. Further, said hydraulic/spring-system is not suited to fulfill the requirements of a maximum length of the valve if it were to be used on for instance a surface test tree.
U.S. Pat. No. 2,780,290 describes a check valve suited for use in a drill string. This valve is intended to be open during normal operations, and is closed in the case of for instance a blowout. The valve is not suited to fulfill the requirement of a maximum length if it were to be used on for instance a surface test tree.